Multiplex carrier current communication system with transmission line impedance control means



May 27 1947- L. KATcHAToURoFF ETAL 2,421,333

MULTIPLEX CARRIER CURRENT COMMUNICATION SYSTEM WITH TRANSMISSION LINE IMPEDANCE CONTROL MEANS Filed April 13, 1943 5 Sheets-S1186*l 1 ATTolrNE May 27, 1947- L. KATcHAToURoFF ETAL 2,421,333

MULTIPLEX CARRIER CURRENT COMMUNICATION SYSTEM WITH TRANSMISSION LINE IMPEDANCE CONTROL MEANS 5 Sheets-Sheet 5 Filed April 13, 1943 May 27, 1947- l.. KATCHATOUROFF ETAL 2.421,333

MULTIPLEX CARRIER CURRENT COMMUNICATION SYSTEM WITH TRANSMISSION LINE IMPEDANCE CONTROL MEANS 5 sheets-sheet 4 Filed April 15, 1945 NES" Qkwm bb, mmv

QE S S May 27'y 1947. l.. KATCHATOUROFF E'r AL 2,421,333

MULTIPLEX CARRIER CURRENT COMMUNICATION SYSTEM WITH I TRANSMISSION LINE IMPEDANCE CONTROL MEANS Filed April 1s, 194s 5 sheets-sheet s FREQUENCY BAND FILTER CHANGE@ AMPL/F/EE Patented May 27, 1947 MULTIPLEX CARRIER CURRENT COMBIUNI- CATION SYSTEM WITH TRANSMISSION LNE IMPEDANCE CONTROL MEANS Leon Katchatouroff and Marc A. Lalande, Lyon, France, assignors to International Standard Electric Corporation, New York, N Y., a corporation of Delaware Application April 1-3, 1943, Serial No. 482,906

In France April 17, 1941 16 Claims. (Cl. 179-15) The present invention relates to carrier current communication systems and more particularly to carrier current multiplex telephone systems.

One of the objects of the invention is the pro,- viding of multiplex transmission systems such as carrier current multiplex telephone systems in which the terminal equipment devices, instead of being located at the two ends of the connection, may be disposed along the same in such a way as to be able to hook up the desired number of carrier current channels at certain points of the circuit as determined by traiic requirements.

Another object of the invention is the providing of multiplex transmission systems in which there are certain elements which are individual to each of several channels or to several groups of channels in the terminal equipment and in which all the channels can be transmitted over one only circuit in one direction and over another circuit in the reverse direction, or else in which the transmission can be effected in both directions over the sameA circuit, in which case one band of frequencies is assigned for the transmission in one direction and another band of frequencies for the transmission in the other direction.

Another object of the invention is the providing of multiplex transmission systems such as carrier current multiplex telephone systems which can furnish circuits that can be directly utilized in central battery or local battery automatic telephone networks.

Another object of the invention is the providing of multiplex transmission systems such as carrier current multiplex telephone systems in.

which the operationsy of signalling, supervision and/or remote control do not require the transmission of frequencies that are solely assigned to these duties. i

Another object of the invention is the providing of multiplex transmission systems, such as multiplex telephone systems, which can have their terminal installations Vstandardized in a way as to facilitate their manufacture and replacement.

According to certain characteristic features of the invention, independent terminal equipment devices are disposed along a multiplex transmission circuit such as a coaxial cable, the hooking up of the said terminal equipment devices to the said circuit being effected in such a way as not to modify its transmission characteristics. The transmission between any twoequipment apparatuses along the circuit is accordingly not disturbed by the branching of other terminal equipment devices on the circuit.

Certain channels furnished by carrier current CII systems can be utilized for connecting 11Dk a certain number of subscribers to an automatic or a central battery exchange. In this case, the equipment devices at the exchange end comprise means that permit reception and transmission of the customary exchange signals, while theA terminal equipment devices that are, at the subscriber end comprise means that prmit connection to the individual telephone sets. Some of the channels. may also be provided so as to form station t0 station connections between twoV local battery stations. o

According to other features of the invention, the coupling of the terminal equipment devices t0 the. Ciruuit is effected by means, of liue repeat,- ers. According to the number of channelsoone or more terminal equipment devices are branched 011 theV repeater Station- The emission terminals of the terminal equipment are connected to the input of therepeater and the reception terminals of the terminal equipment are .Connected t0 the, Output ofthe repeaters., Aurdngly, the channel CUYISIIS @hat l'ech as group Of terminal equip' ment devices are previously ampiied @md the channel currents that leave the group of terminal equipment devices are likewise amplified before being sent over the line.

According to another feature of the invention, use iS. made 0f a. repeater with attenuated gain as coupling means for a group of terminal equipment devices when this group of terminal equipment devices is to be branched at an intermediate point of a section of repeaters.

According to other features of the invention, each section of the transmission circuit is looped on its adapter impedance. This impedance is formed by the connecting in parallel of the input or output impedance of the repeater and the terminal transmission or reception impedance represented by the totality of all the terminal equipment devices branched/t0 the repeaterstttion under consideration. This terminal impedance is provided by the impedance that results from the connecting in parallel of the input or output circuits of all the terminal equipment devices and by an additional adjustable resistance whose value is determined as a function of the number of channels. y

According to another feature of the invention, the carrier current during the communications is transmitted for each channel in an attenuated condition for the purpose of not overloading the line repeaters, and it is then amplied in the terminal 'reception equipment so as to serve for lill@ detection of rthe modulated currents received.

This obviates the difficulty of the local generation of carrier frequency with such degree of precision as not to vary by more than a permissible margin from the carrier frequency at the transmitting end, and this is particularly difficult to obtain in the system under consideration on account of the high values of the carrier frequencies.

According to another feature of the invention, the attenuated carrier current that is transmitted for each channel for the requirements of detection at the receiving end is likewise utilized for signalling, e. g., for manual or automatic ringing, as well as to insure the maintenance of communication in the case of an automatic network.

According to another feature of the invention, a carrier frequency of rst modulation is generated by individual oscillators in the equipment devices of each channel. In systems in which all the channels are independent, their frequency is the same for all the channels. In systems in which channels are associated in groups, there are as many dilferent oscillator frequencies of first modulation as there are ch'annels per group, since these groups are themselves independent of each other; the same oscillators are found in the diiferent groups. This arrangement retains the independence of the channels or groups of channels and at the same time permits standardization of the carrier frequency instruments of first modulation of the various channels or various groups of channels. In the case of an independent channel system, one of the bands of rst modulation, e. g., the upper band, is modulated a second time by carrier frequencies of higher values. In the case of a system of groups of independent channels, the side bands of rst modulation of all the channels of the group are modulated together by carrier frequencies of higher values. In this second modulation, a different carrier frequency value is assigned to each channel in the case of the independent channel system and to each group of channels in the case of the system of groups of independent channels, and this makesit possible to effect separation of thechannels or groups of channels as the case may be. Y Y

According to one feature of the invention, in a terminal installation there is retained from the rst modulation a side modulation band and an attenuated current of the carrier frequency which are transmitted over the line after the second stage of modulation. At the receiving end, after the iirst stage of demodulation the attenuated carrier current of the first modulation frequency is separated from its side band. Itis then amplied to a level that is sufficiently high with respect to the level of its side bands for use to be made of itfor the demodulation of the latter in a demoduilatin'g stage.

VTheser objects and characteristic features, as well as others, will be explained in detail in the following description given with' reference to the appended drawings for thecase of an independent channel system. It will however be evident that thedescribed arrangements can find direct application in systems of groups of independent channels as dened above. In these drawings: Fig. 1 is a schematic view of one example of a carrier current multiplex telephone system that makes use of features of the invention,

Fig. 2 is a schematic View of a repeater station that comprises terminal equipment devices according to features of the invention,

Fig. 3 is a schematic view of one example of the terminal equipment of a channel in a carrier current system that incorporates features of the invention,

Fig. 4 illustrates schematically one embodiment of a connection arrangement that comprises the elements of iirst modulation, and

Fig. 5 illustrates schematically one example of an embodiment of a connection arrangement that comprises theelements of second modulation.

Fig. 1, according to certain features of the invention, shows an independent channel multiplex telephone system that comprises a certain number of repeaters I to 5 disposed in succession along a coaxial cable circuit 6 and fed with energy by the cable 6 in well known manner.

Repeater-s i, 2, 3, and 5 are normal repeaters installed at the ends of the cable sections. Repeater 2 does not comprise terminal equipment and, for example, is a repeater without supervision. Groups of terminal equipment devices l, 8 and `9 are respectively connected to the repeater stations I, 3 and 5. At 4 there is shown a repeater of attenuated gain that serves for the branching of the group of terminal equipment devices l at a point of the cable 6 that is intermediate between two normal repeater stations, 3 and 5 in the example illustrated, without the hooking up of this group of terminal equipment devices Hl causing any disturbance in terminal equipment devices. The gain of this repeater l in each direction is adjusted according to the length of the section of line that precedes it, so that all the repeaters of the system may have the same output level. In this case the gains of the adjacent repeaters 3 in the direction E-W and 5 in the direction W--E are correspondingly small in order to maintain the desired value at the equivalent of transmission, the gains of repeaters 3 and 5 in the reverse direction being normal.

The groups of terminal equipment devices of channels 1, 8, Iii and 9, which are respectively branched to the repeater stations l, 3, 4, and 5, are of a number that corresponds to the number of circuits that it is desired to establish between the corresponding repeater points.

One example of distribution of the channels in the network is given in Fig. 1 by the iigures alongside the letter V for the case of a system of independent channels. This example assumes station to station connections, but it must be understood that any desired distribution can be adopted. For example, use can be made of coaxial cable circuits for connecting an automatic telephone exchange to a number of subscribers distributed along the cable. In this case, all the circuits will have one of their ends in the repeater station next to the central exchange and the other in the repeater station next to the place where those subscribers are who are to be connected to the central exchange. The terminal equipment devices at the central exchange end will then comprise the necessary elements for receiving and transmitting signalling circuits proceeding from the automatic exchange, while the terminal equipment devices at the ends next to the subscribers will comprise the requisite elements for hooking up to their telephone sets. The exploitation of the network may also be mixed, some of the channels being reserved for automatic traiiic and some being assigned to station to station communication.

Fig. 2 shows a repeater station which is fed with energy by cable 6 and to which are branched the terminal cquipmentdevices, off. n channels .proceeding `toward the west; and the terminal dey vicesL of m `channels proceedingv toward the east from this station. Two terminal equipment devices for each direction, Il, I2 for the coin-mu'- nicationsfin the westerly direction and t3., IrlA for the` communications in the .easterly direction are shown in the drawing. These terminal equipment :devices are connected on the one hand to 2-wire telephone circuits whose terminalsv are indicated at t5 and I6. All the terminals of the terminal equipment devices in t-he same direction are connected in parallel, and similarly all the receiver terminals. The 'circuits thus connected in parallel `are led .into` the repeater station -properly so-called, the` transmission terminals being connected to the inputs ofthe corresponding repeaters and the reception terminals to the outputs. The output terminals of the terminal devices I3: and lf3, etc. are accordingly connected by a cable I'I tothe input of the repeater I3 in the west-east direction, and the output terminalsA of the devices II, I2, etc. are connected by a cable I9 to the 'input of a repeater 2u in the east-west direction. Similarly, the reception terminals ofthe terminal equipment devices I, I4, etc. are connected by the cable 2i' to the output ofthe repeater 20', while the reception terminals of the terminal equipment devices- II, IZ, etc. are connected by the cable 22 to the output of repeater I8.

The power supply transmitted by the cable is shunted by the feed filters 23 and 2E on the bars 25'and 26. In parallel on these latter are branched the power supply elements indicated con'jointly by the reference number 2l. These elements supply the voltages required for operation of the repeaters I8 and 2t. The currents of the communication frequencies comprised in the band of frequencies assigned to the west-east transmission are shunted inthe fllters 2-8 and 29. Similarly, the communication currents comprised in the b-and of frequencies assigned tothe east-west transmission are shunted. over filters 3i) and 3|. Attenuator-corrector circuits 32 and 33 are installed in front of the repeaters I8 and 2B respectively. The attenuators permit regulation of the gain of the repeater for the corresponding direction of transmission, according to the length of the section that precedes it. In this way use can be made of repeaters of identical construction and intrinsic gain along the entire length of the cable network, the adjustment of the gain at each repeater point being effected by an auxiliary member such as an adjustable attenuator. The correctors serve for correcting the attenuation distortion of the corresponding sections of the cable.

For the purpose of terminating the cable section 6 in the customary way by an adaptor impedance, the repeaters I8 and 20 should have their input impedance and their output impedance equal to the characteristic impedance of the cable. For this purpose, load resistances are to be introduced into the input and output circuits of the repeaters. In the repeater stations that make use of features of the invention, these load resistances are utilized for the branching of the terminal equipment devices on the cable Without introducing disturbance in the transmission of the other channels. For -this purpose, the load impedances consist of the impedance that results from the connection in parallel of the overall impedances represented by the terminal; equipment devices andv impedances-installedin the repeaters.

Fig. 3 illustrates one example of an arrangement of a channel terminal equipment that makes useof features of the invention.

The Z-w-ire telephone; line 34 that is -connected to the terminal equipment of the channel terminates in a termination set 35' that is provided with a balancing network 36.

The voice frequency Currents that rreach the subscriber station pass from the termination set 35 over an equalizing and attenuating network 31 and are applied to a modulator 38 which consists for example of a modulator with dry elements such as copper oxide rectiers. The carner frequency is furnished by a quartz controlled oscillator 39` at 60. kilocycles. Hereinafter this frequency shall be called carrier frequency of rst modulation.v The modulator is of the balanced type in order to insure suppression of the carrier current in its output. The two side bands issuing from the modulator 38 pass through a band filter fill in which one of the side bands is suppressed. Furthermore, the carrier current generated bythe oscillator 39 is reinjected in the output-of this band lter as indicated at 4 I. This insures a stable and controllable level of the carrier current in the energy transmitted for the communication. The level of the carrier current restored to the line is adjusted in such a way as to supply a carrier current energy that is substantially equal to the average energy of theside band.v The side band that is retained is applied at, the same time as the carrier current to an amplifier tube 42 which serves as input tube to the high frequency connection arrangement, and then to a quartz controlled frequency changing tube 43 which furnishes a second modulation at a higher frequency that may for example be between 200 and 2000 kilocycles. The output of the frequency changing tube is then sent through a band lter 44 which eliminates one of the side bands, and the carrier current of this second modulation is led to one of the cables, e. g., Ii,

f that connects the terminal equipment devices to the repeater station. A coupling potentiometer 45 provides the connection between the band filter d'4 'and the conductor I'I while at the same time also permitting impedance matching of the station of repeaters that are provided with terminal equipment devices in the above mentioned manner. 46 indicates the lead-ins of other channels connected by cables I'I or I9 to the station of repeaters.

For an incoming communication, cable ZI (or 22) leads in the communication currents of the different channels of the station which are multipled as shown at lil. The current assigned to the terminal equipment under consideration passes through a coupling and preselection circuit llt. After amplification in the tube i9, a band filter 56 selects the useful modulation band which is applied to a quartz controlled frequency changing tube 5I that effects a first demodulation and sends the carrier current and the side band of first modulation to the low frequency equipment. The carrier current is separated from this modulation band by a, very narrow band filter 52. This filter is, for example, a quartz filter. After selec tion, the carrier current is amplified in the tube 53 and brings into operation the frequency changing tube 54 on which the modulation band selected by filter 55 is applied. At the output of tube 5d there is another filter 53 which insures .elimination of the carrier current in the output Yof the frequency changing tube 54. The transmission current thus demodulated is amplified by the tube l and is then applied to the termination set or diierential transformer 35.

The carrier current, amplied by tube 53 and by the triode portion of the frequency changing tube 54, is then applied to a tube 58 in order to actuate a call receiving circuit that is indicated schematically by relay 59.

The carrier current of first modulation generated by oscillator 39 is likewise used as a signalling current, the sending of which in the line is controlled by the call emitting circuit indicated schematically by relay B0.

Figs. 4' and 5 show in greater detail the first modulation and second modulation connection arrangements already illustrated in Fig. 3. In these drawings, the elements that correspond to the elements of Fig. 3 are designated by the same reference numbers.

First referring to Fig. 4, the termination set 35 of the 2-wire telephone line 34 is connected to a channel equalizer El of any suitable type of construction by a transformer 62. At the output of this equalizer there is installed an attenuationpad 63, of T shape in the illustrated eX- ample, on account of the nature of the dry rectinerv bridge modulator 38 to which it is connected. The input connections of this bridge modulator 38 comprise a self-inductance 92 of such dimensions that it prevents the return of the side band components to the low frequency terminals without interfering with passage of the low frequency currents to the modulator. At the output of the modulator 33 is located a band filter 49 made in such a way that it presents a large input impedance for the low frequency components while allowing passage of the useful modulation side band.

The oscillator tube 39 comprises a grid-plate coupling device which consists of a piezo crystal 64 that has one of the electrodes divided in two, with one half branched in the plate circuit and the other in the grid circuit of tube 39. The tuned transformer 65 feeds the bridge modulator 38 with carrier current. Owing to the balancing of this modulator, the carrier current becomes substantially suppressed after modulation. It is reinjected by a separate circuit 66 extending from the plate circuit of tube 39 to the terminals of the last section of the band filter 49. Reintroduction of the carrier in the input of the lter would have a drawback. At the frequencies next to the carrier frequency, the nlter represents an abrupt cut-out and, as a result of this, the attenuation provided by the lter to the carrier 'current would essentially be a function of the exact value of the frequency of the latter, or even of the precision of construction of the filter.

A normally closed contact of a signalling relay that forms part of the signalling circuit 60 (the details of which are not shown) is connected by the wires 61 to the output terminals of the band filter 45, and consequently of the rst modulation connection arrangement. This contact shortcircuits the output terminals in order to prevent transmission of the carrier current over the line when the circuit is not in use (this in the case of a central battery or automatic system). During the call signalling operations, this contact opens and closes at the cadence of the signals that are being transmitted. The carrier current is transmitted over the line at the cadence of these openings in order to actuate at the other end the corresponding signalling elements and, on

termination of the signalling, the contact remains open during the entire connection period and thus permits transmission of the carrier for therequirements of the demodulation at the receiving end.

In the emission circuit of the second modulation connection arrangement illustrated in Fig. 5, the side modulation band and the reinjected carrier current are applied to the amplifier tube 42 by means of the potentiometer 68. After ampliiication, the voltage is tapped at the terminals of the anti-resonant circuit 69, which is inserted in the plate connection of tube 42 and damped by the resistance 19, in order to be applied to the control grid of the frequency changing tube 43. This tube comprises an oscillator element stabilized by the quartz crystal 1I. The crystal 'H is installed in a thermostatic enclosure 12 for the purpose of keeping its temperature constant, e. g., at i2 C., for example. The same thermostatic compartment also contains the quartz crystal 13 that is intended for the high frequency demodulation stage 5I. In the plate circuit of the frequency changing tube 43 there is inserted a band lter 44 that permits passage of the upper band of second modulation and suppresses the lower band and the carrier current. This iter is designed in such a Way as to insure particularly rigorous suppression of the said carrier current of second modulation. As a matter of fact, if this current were imperfectly suppressed, it would cause operation of the signalling devices of another channel. This other channel is located in the range of frequencies used for the complete system at such a height that its carrier current of first modulation that insures the signalling of this channel furnishes after the second modulation, a frequency that is equal to the second modulation frequency under consideration.

The output circuit comprises a potentiometric device having one part consisting of the high value series resistance 45, and having its other part, at the terminals of which' the output voltage is tapped, consisting of the impedance that results from the connecting in parallel of the cables impedance (shown at 14), of the repeaters input impedance (shown at l5), of the impedance equivalent to the impedances of the emission circuits of all the terminal equipment de- Vices branched at this point (which are shown at 46), and of the additional resistance 16, the value of which is determined according to the number of equipment devices connected in parallel at this point.

There is thus insured a correct termination of the cable section and there is furthermore obtained, toward the terminals 'Il andl in the direction of the arrow, a constant impedance that is moreover equal to half the characteristic impedance of the cable irrespective of the number of terminal equipment devices connected in at this point. Accordingly, the high frequency side band level emitted toward the terminals Il, I8 becomes likewise independent of the number of terminal equipment devices branched at this point. The signals emitted in this way are transmitted along the cable to the station that comprises the terminal equipment devices that are to receive them.

The arrangement and the operation of the receiving elements of a channel may be described as follows, with' reference to Figs. 5 and 4.

The coupling between the cable and the high frequency reception equipment is effected at the acci-cae output terminals of therepeater by means of the impedances 4l of the receiving circuits of the terminal equipment devices branched at this point and of the additional resistance 8|, which plays a part similar to that of resistance 16 located in the emission circuit. 19 indicates the characteristic impedance of the cable and 3| theV output impedance of the repeater. The result obtained by this arrangement is, on the one hand, that the cable is correctly terminated irrespective of the number of terminal equipment devices branched at this point and, on the other hand, that the voltage received at the terminals 82 `and 83 is independent of the .number of terminal equipment devices branched at this point.

Circuit 158 is a preselector circuit of low selectivity that isprovided for preventing an overload of the amplier tube i9. The output of tube 49 is applied to the frequency changing tube .f5-I

by means of a bandfilter il.

This lter permits passage .of the desired high frequency band and the adjacent bands. On the other hand, it sh'ould completely .eliminate certain other high frequency bands transmitted by the cable which, after demodulation in .tube 5l, might furnish components contained .in the spectrum of first modulation frequencies of the channel under consideration. This would' of course result inunpermissible .cross-talk. It is evidentin the system described as an example, in which the secondmodulation upper band istransmitted over the line, that vthe high frequency bands to be eliminated at =.the receivingend .are those whose differencein frequency below the received bandis aboutltwice .the .value `of ,the carrier frequency of rst modulation. The other part vplayed by the band filter v50 is .that of preventing the overloading of .demodulating vtube 5l.

Demodulating tube5l has its oscillating triode portion stabilized bythe .quartz crystal 13. The variable condenser 84 permits slight modification ofthe valueof the oscillationfrequency.and :also furnishes a means for'making upforthe deviations of frequency that may occur in the various modulation stages .both at the sendingand receiving ends. The output terminals of tube 5| are connected to the input terminals ofthefirst modulation vnetwork (Fig. 4) in rwhich filter 55 is for the purpose of permitting `passage ofthe side band of first modulation vthat is obtained at the output of demodulating stage 5I.

Inshunt to lter 55 thereis installed a quartz lter52 which selects thecarrier current. This filter consists .substantially of the quartzcrystal 52 connected to resistance 85 and to the damped anti-resonance circuit 85. It thus vpermits passage .of rthe carrier frequency of rst modulation While considerably attenuating all the components of the adjacent band of `first modulation. The above mentioned adjustmentof condenser 34 permits coincidence, with a slight tolerance, of the frequency of th'e carrierA current that is received from thedemodulating tube 5l with the tuning frequencyof thequartz52. The carrier voltage of first modulation picked up at ythe terminals of the smoothing circuit 8S Vis applied to the reaction amplifier tube 53. At theterminals Vof the plate circuit of tube 53 :there `is tapped a carrier frequency voltage of rst modulation that is suiiiciently high for its application to the first grid of the demodulating tube 54 to be able to insure correct operation of the'latter. The side band components .of first modulationproceeding from the band filter 55 Yare i@10 appliedtothe-controlgridof tube 5,4. The audio frequency components picked up -in `the plate Ycircuit of tube v54 .are transmitted to the output amplieritube 5.1 ,through the filter unit 5.6, which .is for eliminating :the carrier current, and the adjusting potentiometer 81. Winding 88 of the differential transformer 35 is inserted in the plate circuit of tube 5'!` and it insures transmission of the audio currents to the 2-wire line. In order to prevent a fraction ofthe carrier current from remaining on th'e 2-wire line. or else bypassing transformer 35 to 4the Vcircuit of transformer v62 Vand thereby yinterfering with the operation of the signalling devices of the channel in'question `or of any other channel that might be in tandem .with the same, a more rigorous elimination of the carrier current is insured by the .selective .counter-reaction circuit installed inthe cathode ,connection Yof tube 5-1 and by the series resonant circuit r9i] connected in parallel on th'e winding `83.

The carrier voltage of first modulation is tapped .to .tube 58 which is intended for operation of Vthe signalling relays contained in the ,circuit which is indicated v.in general by the reference numbers 15.9-.6 0.

These.` signalling .circuits E9-60 are not ,described in detail, sincetheir construction vis .not specifically a part ofthefpresent invention. It must be `understood :however that they have to be adapted .to the variousoperations of call signallingrequired by theisystem. Their construction willconsequently vary .according to the type of network in vwhich they are to be used. vFor example, vit :is .evident .that their construction will be -different ,for manual or automatic telephone networks. and, .in @the case of an. automatic telephonesystem, according to whether they iare intended for a subscriber stationor fora central exchange.

`Apart ifrom the construction .of the signalling circuits, the .first .modulation frequency connection arrangements canevidently beidentical for all f the channels, and "this permits .their standardization.

vAll the arrangements vrdescribed above with. reference to Figs-.3 .to:5; andrelating to anindependent channel system `apply in their entirety to the case zojf a 'system of Lgroups .of .independent channels. Taking .asbasis .the independent channel Vsystem :described zaboveV :as van example, :a systemof groups 10i independentazchannels can be obtained,V forexample, by connecting in. groups of :five .the -rst .modulation .connection arrangements of the independent channels. .These first .modulation inetworks rthen comprisedifferent oscillators andfrequency filters for the .five channels ofone samegroup. The same elements are .foundinall thegroups. 'In each group the rst vrnodulationgnetworks f connected in this -Way are connected :toza,..c.ommon second modulation network. Comparison `of 'these two `systems show immediately thatffor lfflvechannels there isa saving .of :eight second modulation networks. out ofthe ltotal of `:201.networks zthat were necessary ini' the: independentl channel lsystem. The Asystem proves economical and is capable of standardization because identical connection Aarrangements are found in'allthe groups. p y y Furthermore, infthe described example ofthe system,.a `considerable saving is effected by the simplified :type Iof .1 construction ofthe band lters. These 'viiltersachieily comprise :condensers ,1,1 and self-inductances of conventionalvcommercial types. The quality of-'the ltering obtained in the iilters made in this way has led to the adoption of a value of 6 kilocycles for the spacing of the carrier frequencies of second modulation. It must be understood however that this spacing is not specific to the system but may be selected of any desired value. In this case the iilters should have suitablecharacteristics to conform to the selected spacing value.

The modulation bands may likewise be made wider so as to permit transmission of broadcasting signals over the carrier current circuits provided by the present invention. A

It is evident that the inventionis not limited to the specific examples of embodiment shown and described, but is on the contrary capable of numerous modifications and adaptations without departing from its scope, particularly as regards the frequency band values mentioned and the specific types of construction of the oscillators, filters, modulators and amplifiers shown in the drawings.

What is claimed is:

1. A multiplex telephone system comprising a transmission circuit adapted to pass a plurality of frequency bands, means for applying thereto a plurality of frequency bands and including a two-way repeater, at least two terminal stations disposed along said circuit and connected to said repeater, means for coupling said terminal stations to said circuit, said coupling means comprising impedance control means for correlating the gain of the associated repeater with the transmission circuit impedance, said repeater having an attenuated gain control and means for modulating said repeater by said` frequency bands, whereby they overall transmission characteristics of said transmission circuit are substantially independent of the number of terminal stations coupled thereto.

2. A multiplex telephone circuit comprising a plurality of terminal stations, means for receiving and transmitting intelligence at each of said terminal stations, a main transmission circuit adapted 'to pass a plurality of frequency bands, means for coupling a plurality of intermediate stations to said transmission line, said coupling meansincluding two-way repeaters, and means comprising a potentiometric control connected in theV output lof said repeaters, for adjusting the impedance between said intermediate stations and lsaid transmission' line, whereby the optimum transmission characteristics of said transmission line are preserved substantially independently of the number of intermediate stations connected to said line.

3. In a multiplex telephone system a, plurality ofterminal stations, each having transmission terminals and receiving terminals, each said output circuit having impressed thereon a carrier wave, a rst modulating means and a second modulating means, said rst modulating means comprising means'for impressing voice modulation upon said carrier wave, said second modulating means comprising means for changing the frequency of the carrier to a higher frequency, a trunk transmission line, two-way repeaters disposed along said trunk transmission line, the output of said second modulating means being coupled to the input of one of said repeaters, said repeater having an attenuated gain. control, means for modulating said repeater by the relatively higher frequency output of said second modulating means, and meansfor' adjusting the output of said second modulating means and in correlation with the gain of said repeater to make the transmission characteristics of said line the same whether said stations are connected to or disconnected therefrom, said adjusting means comprising a potentiometric control connected in the output'of at least one of said repeaters.

4. In a multiple telephone system a plurality of terminal stations, input and output circuits in each of said terminal stations, said input circuits comprising a first demodulator, a second demodulator, a trunk transmission line serving all said terminal stations, a two-way repeater disposed in said trunk transmission line, said input circuits being coupled to the output of said repeater, means for controlling the gain of said repeater, and means for correlating the impedance of said output circuit with the line impedance and the number of terminal stations coupled thereto so that the transmission characteristics of said line are substantially unaffected by the number of coupled stations, said means for correlation including in said repeater an attenuated gain control, and also including a potentiometric control connected in the output circuit of said repeater.

5. A telephone communication system comprising means for generating a plurality of subcarrier'waves, means for modulating each said sub-carrier wave by an independent audio frequency wave, means for generating a main carrier wave of greater frequency than said sub-carrier Waves, means for modulating said main carrier wave by each of said sub-carrier Waves, a coaxial cable, means for feeding all of the modulated Waves thereinto, reception terminal equipment having characteristic impedance and connected to said cable, demodulation means in said reception terminal equipment whereby said independent audio frequency Waves are dis-associated from each other, and also including potentio- Ymetric means connected between said means for generating said main carrier wave of greater frequency and said means for feeding said main carrier Wave into said coaxial cable, and adjusted to control the impedance of the output to match said terminal characteristic impedance.

6. A telephone communication system comprising means for generating a carrier Wave, means for audio frequency modulating said carrier wave, said modulation resulting in the production of two side bands, means for suppressing one of said side bands, means for modulating said carrier wave a second time, said second modulating wave being of a higher frequency than said carrier wave, said second modulation resulting in the production of two side bands, means for eliminating one of said side bands, a transmission line adaptedV to pass a plurality of frequency bands, amplifying devices along said transmission'line, the output of said second modulation means being electrically coupled to the input of one of said ampliiiers, means for matching the impedance of said transmission line and said modulating circuits, and means controlled by said second modulation for varying the gain of saidramplier.

7. A telephone communication system comprising a transmission line' adapted to pass a plurality of frequency bands, control stations connected to said transmission line, said control stations comprising exchange terminals and repeater stations, a pluralityY of subscriber stations, each of said subscriber stations 'and' exchange terminals being electrically connected to said transmission line through one of Ysaid repeater stations, means at said subscriber stations and exchange terminals for generating carrier waves of similar frequency, rst means located at the exchange terminals and repeater st ations for modulating said carrier waves, second means for impressing asecond modulating wave on said already modulated carrier, said second modulating wavebeing of greater frequency than said carrier, the frequency of said second modulating wave Vbeing different in each vof said subscriber stations and exchange terminals, impedance matching means for each of said subcriber stations and exchange terminals, and means controlled by the energy of said second modulating wave to match the irnp'edance of said transmission line with that of the circuits of said subscriber stations and exchange terminals. v'

'BQ multiplex telephone communication system comprising a transmission line adapted to pass a plurality of 'frequency bands, coupling means along said transmission line, said coupling means comprising line repeaters, a group of subscriber stations coupled to said transmission line through one of said line repeaters, means in each of said subscriber stations for generating a carrier wave of different frequency, means for audio frequency modulating said carrier wave, means for generating a wave having a frequency higher than said carrier wave, means for modulating a second time said carrier wave by said higher frequency wave, the frequency of said higher frequency wave being the same for each of said subscriber stations; an additional group of subscriber stations coupled to said transmission line through another of said line repeaters, means in each of said last-mentioned plurality of subscriber stations for generating a wave of different frequency, means for first audio frequency modulating said carrier wave, means for generating a wave having a higher frequency than said carrier wave, means for modulating a second time said carrier Wave by said higher frequency wave, said second modulating wave of higher frequency being of the same frequency for each of said last-mentioned subscriber stations; the frequency of the modulating wave of higher frequency at the first-mentioned group of subscriber stations being dinerent from that of the modulating wave of higher frequency in the second group of subscriber stations, means controlled by said modulating waves of higher frequency for controlling said repeaters, each of said modulating waves to control one of said repeaters.

9. In a communication system, means for modulating different sub-carrier waves by different pieces of intelligence to be transmitted, means to suppress one side band of each modulated subcarrier, means to simultaneously modulate a main carrier wave by each of the unsuppressed modulated side bands of said sub-carrier wave, terminal and repeater stations between and through which said waves are transmitted, and means providing attenuation and gain with respect to said waves at junction points between sections of said system such that the intercoupled impedances of said sections are correlated with the characteristic impedance of the adjacent sections of the system, said last means comprising potentiometric control means acting to match the respective output impedances of said sections with' the characteristic impedance presented by the system therebeyond, including all repeater and terminal stations connected thereto.

10. In a communication system, means for modulating different sub-carrier waves by different pieces of intelligence to be transmitted,

means tosuppress one-sidefband of each modulated sub-carrier, means to simultaneously modulate amain carrier wave by each of the unsuppressed modulated side bands `of said sub-carrier waves, 'terminal and repeater stations between and through which said waves are transmitted, a mixer stage having input and output terminals, located at each terminal station, means for suppressing said main carrier wave as between the input and output terminals of a mixer stage in a given terminal station, and means for reinjecting said main carrier wave into the output energy from said stage, said carrier suppression means including control means operated by said main carrier wave.

'11. In a communication system, means for modulating diierent sub-carrier waves by different piecesof intelligence to betransmitted, means to suppress one side band of each modulated subcarrier, means to simultaneously modulate a main carrier wave by each of the unsuppressed modulated side bands of said sub-carrier waves, terminal and repeater stations between and through which said main carrier wave are transmitted, a mixer stage having input andfoutput terminals, locatedat each terminal station, means for suppressing said sub-carrier wave as between the input and output terminals of a mixer stage in a given terminal station, and means for reinjecting said sub-carrier Wave into the output energy from said stage, said carrier suppression means including control means operated by said main cart rier wave.

12. In a communication system, means for modulating different sub-carrier waves by difierent pieces of intelligence to be transmitted, means to suppress one side band of each modulated subcarrier, means to simultaneously modulate a main carrier wave by each of the unsuppressed modulation side bands of said sub-carrier waves, terminal and repeater stations between and through which said Waves are transmitted, a plurality of transmission branches extending individually from one of said repeater stations to different subscriber stations, and means providing compensation for diiferences in the impedance characteristics of said branches, whereby the impedances thereof are matched, said last means comprising potentiometric control means matching the impedance characteristic of each output circuit at a terminal and repeater station with the over-all impedance of the system therebeyond, including all additional repeater and terminal stations.

13. A multiplex system according to claim 1 in which' the input and output impedances of the repeater are respectively made equal to the characteristic impedance of said transmission circuit by means of input and output load resistors.

14:. A multiplex carrier telephone system including signal-controlled circuits, means for producing control impulses, a plurality of signal-carrying voice frequency lines, a common high frequency transmission line for interconnection, means to produce carrier waves of differing frequencies, means to convert the signals from a voice frequency line into successively modulated carriers, thereby producing side bands, the last carrier being of the higher frequency and being modulated by a single side band of the rst carrier frequency to control repeater operation, means for suppressing said carriers, means for applying said modulations to a carrier wave of the lower frequency so as to produce signals associated with ea'ch voice frequency line for controlling the setting up of'telephone connections and the like, said signals being used to short cir-v cuit the unmodulated carriers from the line when the line is not in use, and to connect said suppressed carrier in cadence with said control impulses to control the establishment of a telephone connection between the voice frequency lines.

15. A multiplex system according to claim 14 comprising means for suppressing the second higher frequency carrier so as to prevent false operation of said signal controlled circuits.

16. A multiplex system according to claim 14 comprising means for preventing any appreciable portion of the higher frequency carrier from reaching the voice frequency line to cause false operation of said signal controlled circuits, the lastmentioned means comprising a grid-controlled ampliier tube having a reaction circuit connected in its cathode return.

LEON KATCHATOUROFF. MARC A.

REFERENCES CITED The following references are of record in the le of this patent:

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